In arc welding, a depression forms in the molten pool from the pressure of the arc and the material transferred by the arc from the electrode to the work-piece. This depression is often referred to as the crater. At the end of the welding operation, this crater remains in the shape of an end crater. When the energy supply ceases abruptly at the end of the welding operation the molten material solidifies while shrinking. The solidification starts at the weld edges and progresses towards the centre. Particularly in the case of high-energy supply welding with consequential large molten pools, there is a deficiency of material in the centre of the molten pool as the molten material solidifies. As a result, a so called pipe forms, i.e. a hole extending vertically through the end crater. Often, this pipe also is the origin of fissures or cracks extending in the longitudinal direction of the welding bead. Welding defects of this nature impair the weld strength and for this reason various methods have been tested in order to eliminate the end crater.
It is known that in manual welding operations the welder often fills up the end crater by increasing the speed of advancement immediately prior to the end of the welding operation, whereupon he moves the gun back in the molten pool in order to thus produce the crater filling. The method puts considerable demands on the skill of the welder-and its repeatability is poor.
It is further known in automated arc welding to extinguish the arc for a programmed period of cooling and to thereafter re-ignite under reduced welding data conditions in order to produce the crater filling for a programmed filling period (U.S. Pat. No. 4,578,562). The method is time-consuming and disturbing welding spatter occurs in connection with the re-ignition.
It is finally known to successively reduce the size of the molten pool by continuously or in essentially equally large steps reduce the welding voltage and the wire feed rate, respectively. Particularly when the welding is performed under high energy supply in the spray transfer range, such as is the case in MIG/MAG welding, the welding data will, in the fading stage, pass through a range wherein considerable welding spatter and an uneven arc are produced.